Abstract

This paper proposes a non-linear distortion noise power control method with bandwidth control for
multiple frequency band transmission which simultaneously uses plural frequency bands in wireless
communication systems. The control method employs clipping and filtering, and generates out-of-band
noise reduction signals using a part of used signal bands to reduce harmful interference in a primary
existing system which shares frequency bands with the multi-band system. The improvement of Signalto-
Noise Ratios (SNRs) in the bands used by the primary system is evaluated by computer simulations.
The simulation results show that the proposed method at the band use rate of 50 % can improve them by
15 dB at the receiving power ratio of -30 dB to the spectrum sharing multi-band system.

1. Introduction

Future wireless communication systems require broader frequency
bands to realize larger capacity because of the rapid spread of
smartphone and the development of IoT (Internet of Things).
Spectrum sharing is one of the promising technologies to yield
broader bands with limited frequency resources, in which plural
systems share the same frequency bands, and the secondary system
uses unused bands in the allocated bands to the primary existing
system [1-4]. Because the unused bands are narrow and separated,
the secondary system needs to simultaneously use multiple bands to
transmit broadband information signals.

On the other hand, OFDM transmission, which is very effective
in broadband transmission in multi-path fading channels, is widely
used in wireless communication systems while it causes excessive
peak power which generates in-band and out-of-band noise because
of non-linear distortion of transmission power amplifiers. In addition,
multi-band transmission with OFDM causes serious distortion noise
by inter-modulation distortion. The distortion noise of a multiband
system becomes harmful interference to the existing system in
spectrum sharing. Therefore, effective power amplification for multiband
transmission has been studied [5-7].

This paper proposes a non-linear distortion noise power control
method with bandwidth control for multi-band OFDM transmission
systems to reduce interference to the spectrum shared existing system
in wireless communications. The control method generates out-ofband
noise reduction signals using a part of used signal bands to
reduce interference. The method uses clipping and filtering (CAF)
for this noise control, which is one of the very effective peak power
reduction methods of OFDM signals [8-10]. The conventional CAF
usually uses out-of-band filtering which reduces out-of-band noise
power caused by clipping. In addition to this filtering, the proposed
method uses in-band filtering which removes in-band distortion
noise components from clipped OFDM signals. This paper clarifies
the effect of distortion noise power control with the proposed method
by computer simulations.

2. Multi-band Transmission Systems

2.1 System Model

Figure 1 shows the system model in this paper. A secondary multi-
band system shares the same frequency bands with a primary existing
one. The service area of the secondary system overlaps with that of
the primary one. Therefore, the transmission signals of the secondary
multi-band system become interference to base and mobile stations
of the primary one.

Figure 1:
Multi-band system model.

Figure 2 shows frequency usage of spectrum sharing systems in
this paper. The frequency bands are allocated to the primary system,
and the unused bands among them are used by the secondary one.
The secondary system simultaneously uses multiple frequency bands
as shown in Figure 2. This use can realize highly efficient frequency
utilization and broadband transmission with limited frequency
resources. However, when multi-bands are used simultaneously,
serious in-band and out-of-band distortion noise occurs by nonlinearity
of a transmitter. The distortion noise increases by using
OFDM transmission employed in current wireless communication
systems because the peak power of OFDM signals is larger than
that of single carrier transmission. This distortion noise interferes
transmission signals on the bands used by the primary system when
the service areas of two systems are overlapped. To realize spectrum
sharing, the secondary multi-band system needs to reduce the
interference so as to satisfy transmission quality for the primary one.

Figure 2:
Multi-band transmission.

2.2 Transmitter with bandwidth control

Figure 3 shows the transmitter for multi-band OFDM transmission
with bandwidth control. Transmission signals are mapped to
modulation symbols and allocated to multiple frequency bands. The
bandwidth of each band is controlled to reduce in-band and out-ofband
distortion noise which is harmful interference of the primary
system. Figure 4 shows the method of bandwidth control for the
secondary system. The method controls the bandwidth used by the
secondary system so as that each used band becomes narrower than
the usable one. This control can generate unused bands in the usable
bands as shown in Figure 4.

Figure 3:
Multi-band OFDM transmitter by clipping and filtering.

Figure 4:
Bandwidth control.

The signals with bandwidth control are transformed into OFDM
signals by IFFT in Figure 3(a). The peak power of the OFDM signals
is larger than saturation power of the used power amplifier, and it
results in much distortion noise out of transmission signal bands.
Then, before power amplification, clipping and filtering, CAF, are
performed to reduce such peak power of OFDM signals.

2.3 Clipping and filtering

The CAF part is shown in Figure 3(b). At the clipping and peak power
detector, peak signal components exceeding the set clipping level in
OFDM signals are detected in the time domain as shown in Figure 5.
These detected peak signals are equivalent to removed components
by usual clipping. The detected components are transformed into inband
and out-of-band clipping noise of the frequency domain by FFT.

Figure 5:
Clipping of OFDM signals.

Figure 6 shows filtering for clipped OFDM signals in this paper. The
filtering part in Figure 3(b) removes the clipping noise. Two methods
can be used as this filtering. One is out-of-band filtering which
removes clipping noise generated outside usable bands by intermodulation
distortion as shown in Figure 6(a). The other is in-band
filtering which removes clipping noise added on the used signal bands
[11]. The in-band and out-of-band filtering leaves clipping noise in
only unused bands as shown in Figure 6(b). The remained clipping
noise becomes out-of-band noise reduction signals.

Figure 6:
Filtering of clipped OFDM signals.

The filtered signals are transformed into peak power components
of OFDM signals in the time domain by IFFT in Figure 3(b). These
transformed signals are subtracted from the original OFDM signals.

Although the peak power of OFDM signals are reduced by this
CAF, the peak reduction becomes imperfect because in-band and
out-of-band filtering removes a part of clipping noise. To reduce
peak power to the set clipping level, the CAF is repeated. These peak
power reduced OFDM signals by iterative CAF are modulated by a
quadrature modulator and amplified by a non-linear power amplifier.

2.4 Frequency band usage

Figure 7 illustrates used band allocation methods in one usable
band for the multi-band system. There are 4 types in the methods.
These differ on the allocation of unused bands in which out-of-band
noise reduction signals are generated by iterative CAF.

Figure 7:
Band utilization methods in a multi-band system.

Type 1 sets unused bands outside the usable band, and the number
of its used band is 1. Type 2 sets one unused band inside the usable
band, and its used bands are divided into 2. In Type 3, the usable band
is first divided into 2, and the same allocation as Type 1 is performed.
This results in 2 used bands and 3 unused bands. Type 4 also divides
the usable band into 2 bands, and the same allocation as Type 2
is employed. Type 4 has 3 used bands and 2 unused bands. In the
following, the distortion noise reduction effects with these allocation
types are evaluated.

3. Simulation Results

3.1 Simulation condition

Computer simulations were conducted to clarify the effect of the
proposed method in multi-band transmission with OFDM. Spectrum
properties and SNR improvement with the proposed method were
evaluated in a receiver of the primary existing system.

Table 1 shows simulation conditions in this paper. The modulation
scheme was 64QAM, and the total FFT point number was 16384. The
number of transmission frequency bands, Nb, was set to 1 and 2 for
the primary existing and secondary multi-band systems, respectively.
The sub-carrier number for single band was 600, and it was 1200 for
the multi-band system.

Table 1:
Simulation conditions.

The input back-off values of non-linear amplifiers (NLAs) were set
to be 8 dB for both systems. In this paper, the typical model of an NLA
was used [12]. The non-linear factor of the model was set to be 3. The
clipping level was 3 dB, and iteration number was 5 in iterative CAF
for bandwidth control in the multi-band system.

The ratio of used bandwidth to usable one was set to be 0 to 100 %
for the multi-band system by bandwidth control, and it was 100 % for
the primary existing system.

The received power ratios of the existing system were set to be 0 dB,
-10 dB, -20 dB, and -30 dB to the spectrum sharing multi-band system.

3.2 Spectrum properties by spectrum sharing

Figure 8 shows received spectrum properties of a secondary multiband
system with and without CAF at an existing system receiver.
The band utilization method of Type 1 was used in this figure. The
frequency of horizontal axis is normalized by the sampling rate 1/T of
OFDM symbols. The band use rate by bandwidth control with CAF
was set to be 60 %. The received spectrum of the existing system is also
shown in Figure 8, and the power ratios Pr are set to be 0 dB and -30
dB in Figure 8(a) and (b), respectively. The existing system uses the
band of third inter-modulation distortion caused by the multi-band
system.

Figure 8:
Spectrum properties by spectrum sharing.

Figure 8(a) shows that bandwidth control by using CAF is effective
in the reduction of out-of-band noise by third inter-modulation
distortion. The figure also shows that the occupied bandwidth of outof-
band noise on third inter-modulation band becomes narrower, and
the bandwidth becomes 60 % which is equal to the set band use rate.

In addition, Figure 8(b) shows that the bandwidth control is more
effective in Signal-to-Noise Ratio (SNR) improvement at lower power
ratios of the existing system because interference power from the
multi-band system is decreased by out-of-band noise power reduction.

Figure 9 also shows received spectrum properties with and without
CAF. The band utilization method of Type 1 was used. The band use
rate by bandwidth control was set to be 60 %. The power ratios Pr are
set to be 0 dB and -30 dB in Figure 9(a) and (b), respectively. In these
figures, the existing system uses an adjacent band of the multi-band
system.

Figure 9:
Spectrum properties by spectrum sharing.

Figure 9 (a) shows that bandwidth control by CAF can significantly
reduce out-of-band noise on the adjacent band of the multiband
system, and the noise power reduction of 12 dB is obtained.
Furthermore, Figure 9(b) confirms that the bandwidth control can
improve SNR performance of the existing system at lower power
ratios.

3.3 SNR improvement by bandwidth control

Figure 10 shows SNR improvement of the primary existing system
when the secondary multi-band system employs bandwidth control
by iterative CAF. In this SNR improvement evaluation, the total noise
power N of SNRs is calculated by the following equation.

N=Dp+Ds(1)

where Dp is own distortion noise power of the existing system
caused by its transmitter non-linearity, and Ds is the distortion noise
power received from the multi-band system. The existing system
uses the band of third inter-modulation distortion by the multi-band
system. The band utilization methods of Types 1 to 4 were used in
this figure.

The band use rate of a multi-band system is set to be 0 to 100 %.
The value of 100 % is without bandwidth control, and 0 % means that
the multi-band system uses no frequency band. SNR improvement is
represented by the difference for the SNR at 100 %.

Figure 10 (a) shows SNR improvement evaluation results at the
power ratio Pr of 0 dB. This figure shows that the effect of bandwidth
control is very small at all band use rates. These results are because the
received signal power is sufficiently large, and own distortion noise
power Dp is larger than Ds from the multi-band system. This results
in small influence of distortion noise power reduction by bandwidth
control. There is no difference by allocation types in usable bands of
the multi-band system.

Figure 10 (b) shows SNR improvement at Pr of -10 dB. This figure
shows that bandwidth control is effective in SNR improvement.
It can improve by 2 to 3 dB at the band use rate of less than 60 %.
The difference by allocation types is not much. Because the received
signal power and own distortion power Dp is still large, the reduction
difference of distortion power Ds by allocation types does not appear
to the evaluation results.

Figure 10 (c) shows the evaluation results at Pr of -20 dB. This
figure shows that the improvement of bandwidth control is larger
than that of high power ratios. The improvement values reach to 5 dB
at a maximum. The figure also shows that the selection of allocation
types is effective. Types 3 and 4 can be more reduced by about 2 dB
compared with Types 1 and 2. Because types 3 and 4 divide the bands
of out-of-band noise reduction signals, they can reduce distortion
noise of more frequency. In addition, because used signal bands are
divided, noise by third inter-modulation distortion spreads and its
power per sub-carrier becomes lower. This results in distortion noise
reduction within the band used by the existing system, and the SNR
is improved.

Figure 10 (d) shows evaluation results at Pr of -30 dB. This figure
shows that the improvement of bandwidth control is the same as that
at -20 dB. The maximum improvement value is 5 dB at the band use
rate of 10 to 60 %. At less than -20 dB, distortion noise from the multiband
system, Ds, is dominant in the used signal band of the existing
system. Therefore, the improvement values of SNRs are equal in spite
of power ratio lowering.

Figure 11 shows SNR improvement evaluation results by the
bandwidth control. In this evaluation, the existing system uses
the adjacent band of the multi-band system as shown in Figure 9.
Allocation types of Types 1 to 4 were used in the evaluation. The band
use rate of the multi-band system is set to be 0 to 100 %.

Figure 11:
SINR performance by bandwidth control.
(Adjacent band).

Figure 11 (a) shows the evaluation results at the power ratio Pr of 0
dB. This figure shows that bandwidth control has almost no effect on
SNR improvement of the existing system. The reason is the same as
the case of third inter-modulation distortion band, and it is because
the received signal power is sufficiently large, and Dp is larger than Ds.
The results also show that there is no difference of SNR improvement
by allocation types.

Figure 11 (b) shows evaluation results at Pr of -10 dB. This figure
shows that bandwidth control is effective in SNR improvement of the
existing system at -10 dB. It can improve by 4 dB at the band use rate
of less than 50 % by using Type 1. The reduction values are almost
the same as that of 100 % in which the multi-band system uses no
frequency band. Because the unused band for out-of-band noise
reduction signals is close to the adjacent band used by the existing
system in Type 1, the leakage distortion power from the multi-band
system can effectively be reduced. On the other hand, the used signal
band of Type 2 is close to the adjacent band. Therefore, the distortion
noise power reduction effect of Type 2 is smaller than that of Type 1.

Figure 11 (c) shows the results at Pr of -20 dB. This figure shows
that the improvement of bandwidth control is larger than that of high
power ratios. The improvement value reaches to 10 dB at 50 % at a
maximum. The figure also shows that the difference by allocation types
is very significant in case of the adjacent band. Type 1 can especially
obtain larger SNR improvement compared with other allocation types.
At this power ratio region, Ds is dominant in SNRs, and larger Ds
reduction effect by bandwidth control of Type 1 remarkably appears
in SNR improvement.

Figure 11 (d) shows the results at Pr of -30 dB. This figure shows
that the improvement of bandwidth control is larger than that of -20
dB. The improvement value with Type 1 becomes 15 dB at 50 %. The
value becomes 6.5 dB even at 80 %. In addition, Type 3 can improve
the SNR of the existing system by 6 dB at less than 60 %.

The above results confirm that the SNR improvement of the adjacent
band is larger than that of third inter-modulation distortion band.
This is because the band allocated to out-of-band noise reduction
signals becomes near distortion noise to be reduced.

Although Types 3 and 4 are more effective in distortion noise
reduction in third inter-modulation distortion noise band from the
results of Figure 10, the difference of reduction effect is not large. On
the other hand, the advantage of Type 1 is obvious in the adjacent
band from the results of Figure 11. These result shows that Type 1,
which allocates unused bands outside usable bands, is superior to
other allocation types on distortion noise reduction.

4. Conclusion

This paper has proposed a non-linear distortion noise power
control method with bandwidth control for multi-band OFDM
transmission by spectrum sharing, which uses iterative clipping and
filtering. The method employs in-band and out-of-band filtering to
effectively control non-linear distortion noise power and improve
SNR performance. The evaluation results with the proposed method
show that it can reduce out-of-band distortion noise power of a
secondary multi-band system and improve the SNR performance of a
primary existing system. The results confirm that spectrum sharing is
feasible even in multi-band systems.